The present invention relates to a grease sealed bearing for automobile and a grease used in a device driven by an automobile engine.
A timing belt driven by an automobile engine and a belt for driving an auxiliary machine such as an alternator (AC generator) have belt tensioners to adjust a winding angle of the belt, and to provide adequate tension to the belt. Some of the belt tensioners are in a so-called bevel outer ring type that a rolling surface of a pulley that contacts with the belt is disposed directly at an outer ring of a ball bearing. Many of the belt tensioners are an integrated type that a pulley having a rolling surface and a ball bearing are engaged. In the pulley used in the latter type belt tensioners, when the pulley is rotated by transmission torque from the belt, the outer ring of the ball bearing that is engaged with the pulley is also rotated together therewith.
A demand for lightweight automobiles is increasing these days. This demand urges that auxiliary parts of an automobile have to be compact in size and lightweight. On the other hand, such devices are required to have a high power and a high efficiency. A compact size undesirably induces an decrease in the output of the devices. To compensate the decreased output, the devices have to be rotated at high speed.
In the pulley for rotating the belt that transmits rotating power from the automobile engine, in order to prevent a reduction in the transmission efficiency caused by the compact-sized pulley, it is necessary to form the pulley with a plurality of grooves for engaging power transmission belts to keep the tension of the belts high. The bearing engaged with an inside diameter of the pulley of the belt tensioner, which has a rolling surface contacted with the belt, is therefore required to withstand high-speed rotation and high load. Generally speaking, with a grease-sealed bearing, the life of the grease is shorter than the rolling life of the bearing itself and thus the life of the bearing is determined by the life of the grease. Accordingly, it is required to seal the bearing with a grease which has a stable durability at high temperature including prolonged life under the conditions of high-speed and high load.
When the pulley of the belt tensioner is operated at low temperature, a noise such as a hoot noised may be produced depending on specifications and operating conditions of the pulley. The causes of the noise are not yet specifically clarified. While not wishing to be bound by any particular theory, it is believed that the pulley is resonated by self-induced vibration of a rolling element which is induced by irregularity and unevenness of a grease film, and the outer ring is vibrated in the axis direction (translation motion), resulting in the noise at low temperature.
Even if the grease that can prevent the noise from producing is used on the bearing, the high-speed rotation and high load can cause early brittle flaking on the rolling surface of the bearing. Unlike a flaking occurring on the rolling surface or in its superficial area owing to metal fatigue, this type of flaking indicates a peculiar destructive phenomenon which has suddenly started from a deep area in the bearing. Such flaking may occur in the following mechanism: namely, the bearing vibrates due to high-speed operation; the vibration causes wear of the rolling surface into a mirror surface; this produces a new surface, which acts as a catalyst and decomposes the grease, thus producing hydrogen; the hydrogen penetrates into the steel of the bearing, thus causing embrittlement of the steel.
Japanese Patent Laid-Open Publication Nos. 9-208982 and 11-270566 disclose a grease for use in a pulley comprising a base oil containing a synthesized hydrocarbon oil and an ester oil, and a thickener comprising a urea compound. The grease has excellent high temperature resistance, prevents the noise at low temperature, and withstands brittle flaking under high-speed rotation and high load.
Japanese Patent Laid-Open Publication No. 8-113793 discloses a grease suitable especially for an outer ring rotation comprising 10% by weight or more of an ester oil, and a thickener comprising a diurea compound terminated at their ends with aromatic hydrocarbon groups.
However, the aforementioned greases do not satisfy all of the properties including the prevention of the noise at low temperature, the high temperature resistance, and the prevention of the brittle flaking under high-speed rotation and high load. It is difficult to provide both stability of the oily film at low temperature, and durability of the grease at high temperature.
Even if these greases had excellent properties, there would give rise to a new problem that the greases have extremely increased penetration during a long period of operation under a high shearing pressure. The grease having increased penetration tends to be leaked from the bearing, which may decrease the life of the bearing.
In view of the aforementioned problems of the prior art, an object of the present invention is to provide a grease sealed bearing for automobile that has the high temperature resistance as well as shear stability, and prevents the noise at low temperature as well as the brittle flaking under high-speed rotation and high load.
One aspect of the present invention is a grease sealed bearing for automobile used in a device driven by an automobile engine, in which the grease comprises a base oil containing a synthesized hydrocarbon oil and one or more ester oils at a weight ratio of 0.1 to 0.49:0.9 to 0.51; 5 to 30% by weight of a urea based thickener, and 0.1 to 10% by weight of dithiozinc phosphate based on the whole grease.
As used herein, the term xe2x80x9ca device driven by an automobile enginexe2x80x9d describes a device for adjusting a tension of a timing belt, a belt for driving an auxiliary machine and the like.
According to the present invention, the ester oil has a kinetic viscosity of 30 to 100 mm2/s at 40xc2x0 C., and a pour point of xe2x88x9230xc2x0 C. or less.
According to the present invention, the synthesized hydrocarbon oil has a kinetic viscosity of 3 to 65 mm2/s at 40xc2x0 C., and a pour point of xe2x88x9250xc2x0 C. or less.
According to the present invention, the urea based thickener is an alicyclic urea compound.
The ester oil is used as the base oil of the grease, the urea based thickener and the dithiozinc phosphate are mixed therein at the above-mentioned ratio, specifically, the ester oil has a kinetic viscosity of 30 to 100 mm2/s at 40xc2x0 C., and a pour point of xe2x88x9230xc2x0 C. or less, the synthesized hydrocarbon oil has a kinetic viscosity of 3 to 65 mm2/s at 40xc2x0 C., and a pour point of xe2x88x9250xc2x0 C. or less, and the urea thickener is an alicyclic urea compound, thereby providing the grease having excellent properties at low temperature, while maintaining excellent heat resistance attributed to the ester oil. The use of the alicyclic urea compound provides excellent shear stability.